Layers that paint into their composited ancestor should still appear

This layout test demonstrates a case where they do not.

BUG=403819

Review URL: https://codereview.chromium.org/471073002

git-svn-id: svn://svn.chromium.org/blink/trunk@180292 bbb929c8-8fbe-4397-9dbb-9b2b20218538